Various optical scanners are known for such applications as data storage, bar code reading, image scanning (surface definition, surface characterization, robotic vision), and lidar (light detection and ranging). Referring to FIG. 1, a prior art scanner 50 generates a moving spot of light 60 on a planar target surface 10 by focusing a collimated beam of light 20 through a focusing lens 40. If the assembly is for reading information, reflected light from the constant intensity spot 60 is gathered by focusing lens 40 and returned toward a detector (not shown). To write information, the light-source is modulated. To cause the light spot 60 to move relative to the surface 10, either the surface 10 is moved or the scanner 50 is moved. Alternatively, the optical path could have an acousto-optical beam deflector, a rotating prism-shaped mirror, a lens driven galvanometrically, or by piezoelectric positioners. Scanners also fall into two functional groups, raster and vector. Both types generally use the same types of beam deflection techniques.
Higher-speed raster scanners use either spinning prism-shaped (polygonal cross-sectioned) mirrors or multifaceted spinning holograms (hologons). Performance parameters for these conventional beam deflection techniques are listed in Table 1. The discrete optics in these devices are generally associated with high costs for mass manufacture, assembly and alignment.
TABLE 1 ______________________________________ Performance of Conventional Beam Deflectors for Optical Scanning. Galvano- Hologons Acousto- Polygonal Driven (Trans- Optic Parameter Mirrors Mirrors mission) Deflectors ______________________________________ wavefront /8 at 0.55 /8 at 0.55 /6 at 0.55 2 at 0.55 distortion .mu.m .mu.m .mu.m .mu.m area resolution 25,000 (scan 25,000 (scan 25,000 (scan 1,000 (scan (spot-widths/ lens limited) lens limited) lens limited) lens limited) sec cross-axis error 10 arc sec 1-2 arc sec 10 arc sec 0 (uncorrected) (uncorrected) speed (spot 1 .times. 10.sup.8 2 .times. 10.sup.6 2 .times. 10.sup.7 2.8 .times. 10.sup.7 widths/sec) bandwidth 0.3-20 .mu.m 0.3-20 .mu.m Mono- mono- chromatic chromatic scan efficiency 80-100% 65-90% 90% 60-80% ______________________________________ (from the Photonics Design and Applications Handbook 1993, Laurin Publishing Co., Inc., p. H449)
The performance parameters listed in Table 1 assume different levels of importance depending on the optical scanning application. For raster scanning to cover extended surface areas, the emphasis is on speed, area resolution, and scan efficiency. Wide bandwidth is needed if the surface is to be color scanned. For applications requiring vector scanning of precise paths at high resolution, the optical system typically uses a monochromatic, focused spot of light that is scanned at high speed with low wavefront distortion and low cross-axis error. Optical data storage has been a prime application of this type of optical scanning.
In optical data storage media, information is stored as an array of approximately wavelength-size dots (bit cells) in which some optical property has been set at one of two or more values to represent digital information. Commercial read/write heads scan the media with a diffraction-limited spot, typically produced by focusing a collimated laser beam with fast objective lens system as shown in FIG. 1. A fast objective lens, one with a high numerical aperture, achieves a small spot size by reducing Fraunhofer-type diffraction. The spot is scanned by moving an assembly of optical components (turning mirror, objective lens, position actuators) over the optical medium, either along a radius of a disc spinning under the spot or across the width of a tape moving past the head. The assembly moves in one dimension along the direction of the collimated laser beam. As the disk spins or the tape feeds, the line of bit cells must be followed by the spot with sufficient precision to avoid missing any bit cells. The fine tracking is achieved by servo mechanisms moving the objective lens relative to the head assembly. An auto-focus servo system is also necessary to maintain the diffraction limited spot size because the medium motion inevitably causes some change in the mean/medium separation with time. Proper focus adjustment is possible because the medium is flat and smooth. Such a surface reflects incident light in well-defined directions like a mirror. Light reflected from the medium is collected by focusing optics and sent back along the collimated beam path for detection.
Scanning by several spots simultaneously is used to achieve high data rates through parallelism in one known system called the CREO.RTM. optical tape system. One scanning device that avoids reliance on discrete optical elements to achieve scanning is described in U.S. Pat. No. 4,234,788. In this scanner, an optical fiber is supported rigidly at one end in a cantilevered fashion. The supported end of the fiber is optically coupled to a light emitting diode or photo diode for transmitting or receiving light signals, respectively. The fiber is free to bend when a force is exerted on it. The fiber can thus be made to scan when light from the light-emitting diode emanates from the tip of the fiber ss the fiber is forced back and forth repeatedly. To make the fiber wiggle back and forth an alternating electric field, generally perpendicular to the axis of the fiber, is generated. The fiber is coated with a metallic film. A charge is stored on the film, especially near the tip, by forming a capacitance with a metallized plate oriented perpendicularly to the fiber axis (optically at least partly transparent). The stored charge makes the fiber responsive to the electric field.
A drawback of this device is the limit on the speeds with which the fiber can be made to oscillate. The device requires a series of elements to move the fiber: an external field-generating structure, a DC voltage source to place charge on the fiber coating, an AC source to generate the external field. Another drawback of this prior art mechanism is the inherent problem of stress fractures in the fiber optics. Bending the fiber repeatedly places serious demands on the materials. Problems can arise due to changes in optical properties, changes in the mechanical properties causing unpredictable variation in the alignment of the plane followed by the bending fiber, the amplitude of vibration, the natural frequency of vibrations, and structural failure. Still another limitation is imposed by the need to place a conductor between the fiber tip and the optical medium to form the capacitance. This places another optical element between the fiber tip and the scanned surface and makes it impossible to sweep the tip very close to the scanned surface as may be desired for certain optical configurations.
Another prior art scanning device is described in U.S. Pat. No. 5,422,469. This patent specification describes a number of different devices to oscillate the end of an optical light guide or optical fiber. One embodiment employs a piezo-electric bimorph connected to the free end of a device to which the free end of an optical fiber and a focusing lens are attached. Reflected light is directed back through the fiber to a beam splitter which directs the reflected light out of the bidirectional (outgoing/return) path at some point along the fiber remote from the source of light. The above embodiment uses a simpler prime mover, a piezo-electric bimorph. However, the need for a focusing lens attached to the end of the fiber, by increasing the mass, imposes difficult practical requirements for high speed oscillation of the fiber. In addition, to achieve very small projected spot size requires a high numerical aperture at the output end of the focusing optics. It is difficult to achieve this with the conventional optics contemplated by the '469 disclosure. Furthermore, the reciprocation of the fiber as described in the '469 patent requires a multiple-element device. Friction between the motor and the fiber can cause changes in the optical properties of the fiber, and mechanical changes in the motor, the fiber, or the interface, that result in changes (which may be unpredictable) in the amplitude of oscillation or the resonant frequency of the motor-fiber combination (which might generate, or be susceptible to, undesired harmonics). Also, the process of assembly of such a combination of a motor and a fiber presents problems. Ideally, for high frequency operation, the device would be very small.
U.S. Pat. No. 3,892,468 describes an optical scanner that employs a large number of fibers to scan a surface. Light from a pulsed laser is directed to a series of beam splitters with each arranged along the path of the beam to direct a portion of the beam to a respective input end of a fiber optic light guide. Each fiber is a different length, so that each takes a different amount of time to emit its respective portion of the pulse at its output end. The output end of the longest fiber is located at one end of an array that is imaged on a target surface by a focusing lens. When a pulse of light hits the beam splitters, the portion of the pulse directed to the shortest fiber by its respective beam splitter takes the shortest time to reach the output end and be imaged on the target. The second portion is emitted by the second-shortest fiber in turn. Because the output ends of the fibers are arranged progressively in a line with the shortest fiber at one and the longest at the other end, the pulse produces a spatial and temporal scan across the target. Although this device has the advantages of requiring no movable parts to perform the scanning, the beam splitters cut the pulsed lasers beam intensity down substantially. Also, each fiber is used for scanning only once during each laser pulse.
Common to all storage/retrieval devices is the need for greater and greater data rates. Increases in speed have been achieved by increasing the speed of scanning. However, there are practical limits, particularly with regard to the writing operation, relating to physical properties inherent in the optical media.
Also common to the applications of optical scanning technology is the need for great precision in the focus of the scanning light source and the return signal.